Seasonal forecast of the Sudden Stratospheric Warming occurrence

The polar vortex in the wintertime Northern Hemisphere can sometimes experience a dramatic breakdown after an associated warming of the stratosphere during so-called Sudden Stratospheric Warmings (SSWs). These events are known to influence the ground weather in Northern Eurasia and large parts of North America. SSWs are primarily generated by enhanced planetary waves propagating from the troposphere to the stratosphere where they decelerate the vortex and lead to its breakdown. According to the Holton-Tan mechanism, the easterly direction of equatorial stratospheric QBO (Quasi-Biennial Oscillation) winds weakens the northern polar vortex by guiding more waves poleward. Recently, we found that during easterly QBO the occurrence rate of SSWs is modulated by the geomagnetic activity. We used the aa-index which is a good proxy for the energetic electron precipitations (EEP) responsible for the indirect effect on ozone. Our model shows that the breaking of the polar vortex is very likely to occur if the geomagnetic activity is weak. On the other hand, during westerly QBO, solar irradiance modulates the SSW occurrence: more SSWs happen under high solar activity.